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Financial instruments - Long-term Energy Derivative Contracts (Detail) - Cash flow hedge
Jun. 30, 2022
MWh
$ / MWh
Illinois Hub, Expiry September 2030  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 4,297,565
Receive average prices (per MW-hr) | $ / MWh 24.54
PJM Western HUB, Expiry December 2028  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 463,336
Receive average prices (per MW-hr) | $ / MWh 30.62
NI HUB, Expiry December 2027  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 2,194,497
Receive average prices (per MW-hr) | $ / MWh 22.86
ERCOT North HUB, Expiry December 2027  
Derivative [Line Items]  
Notional quantity (MW-hrs) | MWh 1,803,817
Receive average prices (per MW-hr) | $ / MWh 36.46